The invention concerns a double-layer type of forming fabric for use in papermaking, cellulose and similar machines. The forming fabric consists of two complete weaves, each one comprising its separate sets of respective warp and weft threads. The first weave, the top cloth, which in the position of use of the fabric faces the material being formed is made from thinner threads than the second weave, the bottom cloth.
In the production of paper in a papermaking machine a slurry of fibres suspended in large quantities of water is discharged in a flow onto a wire or cloth of mesh-like construction. The wire allows through-flow of the water of the slurry but not of the fibres therein. The fibres therefore collect on the upper face of the fabric and are formed into a web or sheet of paper. To avoid marking on the paper sheet being formed as well as loss of fibres, the paper-forming side of the fabric must be a fine-mesh weave. However, fine-mesh wires made from thin threads are less resistent to wear and abrasion and are also less stable than are coarser weaves. The requirements that the paper-forming side of the fabric must meet thus are in conflict with the requirements regarding the opposite side of the fabric, that is, the side that travels in contact with guide rolls and suction boxes in the papermaking machine, where the fabric is exposed to considerable friction and wear. To solve these problems of conflicting requirements double layer fabrics have been constructed that consist of two complete weaves, one of which is a fine-mesh weave comprising the paper-forming side of the fabric, and the other one a courser weave comprising the opposite side of the fabric. These two layers are interconnected either by warp threads that are part of the thread systems of either one of the two weaves, or by separate binder threads. However, the use of a warp thread that is part of the fine-mesh wire layer as the interconnecting thread causes unevenness on the extremely sensitive paper-forming side of the fabric when at regular intervals the interconnecting warp thread interlaces with a thread in the bottom side of the fabric. On the other hand, the use of a warp thread that is part of the coarser wire layer as the interconnecting thread causes unevenness on the paper-forming side of the fabric when this thread interlaces with the fine-mesh fabric layer that faces the paper web. The best method hitherto conceived to interconnect the two weaves is to use separate binder weft threads which are thinner than those making up the fine-mesh wire part. The inherent problem with the use of binder weft threads of this kind is, however, the abrasive effect that these binder threads have on the threads making up the two weaves, resulting in wear on and ultimately rupture of these threads.